In electrophotographic applications such as xerography, a charge retentive surface is electrostatically charged, and exposed to a light pattern of an original image to be reproduced to selectively discharge the surface in accordance therewith. The resulting pattern of charged and discharged areas on that surface form an electrostatic charge pattern (an electrostatic latent image) conforming to the original image. The latent image is developed by contacting it with a finely divided electrostatically attractable powder referred to as "toner". Toner is held on the image areas by the electrostatic charge on the surface. Thus, a toner image is produced in conformity with a light image of the original being reproduced. The toner image may then be transferred to a substrate or support member (e.g., paper), and the image affixed thereto to form a permanent record of the image to be reproduced. Subsequent to development, excess toner left on the charge retentive surface is cleaned from the surface. The process is well known, and useful for light lens copying from an original, and printing applications from electronically generated or stored originals, where a charged surface may be imagewise discharged in a variety of ways.
Developing material commonly used in systems for developing latent images on the charge retentive surface typically comprises a mixture of toner and a "carrier" of larger granular beads of a ferrous material. If the developing system is a magnetic brush assembly, magnetizable carrier beads also provide mechanical control for the formation of magnetic brush bristles so that toner can readily be brought into contact with the charge retentive surface. Toner is attracted to the latent image from the carrier beads to form the toner image. In this type of copier, some carrier beads inevitably adhere to the charge retentive surface after the image is developed. These adhering carrier beads prevent intimate contact between the substrate and toner image during the transfer step. It is thus essential for optimum operation that carrier beads remaining on the charge retentive surface be removed therefrom. Failure to remove carrier beads from the charge retentive surface results in a characteristic copy quality defect displaying a white area with a black dot in the center. The hard carrier beads also have a tendency to abrade and damage the charge retentive surface if not removed prior to reaching the cleaning zone.
Carrier bead removal devices are known, such as for example, U.S. Pat. No. 3,894,513 to Stanley et al. and U.S. Pat. No. 3,834,804 to Bhagat et al., which use a stationary magnet having a cylindrical shell rotating thereabout to remove the ferrous carrier beads from the photoreceptor for deposit in a sump or for return to the developer housing. A primary problem, however, is the need to increase the removal force on the beads without simultaneously increasing the magnetic force holding toner on the shell at the removal or drop off point. In the Xerox 1075 copier, a short piece of magnet was arranged with a polarization opposite to that of the pickoff magnetic to reduce the magnetic field and thus attraction at the bead removal point.
Magnetic rolls for movement of material are known in other applications in electrophotographic devices, particularly for developing the latent image, as shown in U.S. Pat. No. 4,357,097 to Koiso, U.S. Pat. No. 4,303,331 to Thompson, U.S. Pat. No. 4,461,562 to Goldfinch, DE-OS No. 2364 563, JP No. 56-133761A and JP No. 53-29724, all suggesting magnetic rolls for moving developer material into contact with a charge retentive surface. U.S. Pat. No. 4,705,383 to Hiraga et al shows a magnetic held within a U-shaped housing for the purpose of a doctor blade controlling the size of the bristles formed at a magnetic brush.